U.S. patent application number 15/922960 was filed with the patent office on 2018-07-19 for elastic wave device.
The applicant listed for this patent is Murata Manufacturing Co., Ltd.. Invention is credited to Tetsuya KIMURA, Yutaka KISHIMOTO, Masashi OMURA.
Application Number | 20180205361 15/922960 |
Document ID | / |
Family ID | 58556903 |
Filed Date | 2018-07-19 |
United States Patent
Application |
20180205361 |
Kind Code |
A1 |
KISHIMOTO; Yutaka ; et
al. |
July 19, 2018 |
ELASTIC WAVE DEVICE
Abstract
An elastic wave device includes a supporting substrate, an
acoustic multilayer film on the supporting substrate, a
piezoelectric substrate on the acoustic multilayer film, and an IDT
electrode on the piezoelectric substrate. An absolute value of a
thermal expansion coefficient of the piezoelectric substrate is
larger than an absolute value of a thermal expansion coefficient of
the supporting substrate. The acoustic multilayer film includes at
least four acoustic impedance layers. The elastic wave device
further includes a bonding layer provided at any position in a
range of from inside the first acoustic impedance layer from the
piezoelectric substrate side towards the supporting substrate side,
to an interface between the third acoustic impedance layer and the
fourth acoustic impedance layer.
Inventors: |
KISHIMOTO; Yutaka;
(Nagaokakyo-shi, JP) ; KIMURA; Tetsuya;
(Nagaokakyo-shi, JP) ; OMURA; Masashi;
(Nagaokakyo-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Murata Manufacturing Co., Ltd. |
Nagaokakyo-shi |
|
JP |
|
|
Family ID: |
58556903 |
Appl. No.: |
15/922960 |
Filed: |
March 16, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2016/071725 |
Jul 25, 2016 |
|
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15922960 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H03H 9/02133 20130101;
H03H 9/02897 20130101; H03H 3/02 20130101; H03H 9/0585 20130101;
H03H 9/6406 20130101; H03H 9/02559 20130101; H03H 9/02228 20130101;
H01L 41/0533 20130101; H03H 9/175 20130101; H01L 41/1873 20130101;
H01L 41/1871 20130101; H01L 41/0477 20130101; H03H 2003/025
20130101; H03H 9/14538 20130101 |
International
Class: |
H03H 9/02 20060101
H03H009/02; H03H 9/145 20060101 H03H009/145; H03H 9/05 20060101
H03H009/05; H01L 41/047 20060101 H01L041/047; H01L 41/187 20060101
H01L041/187; H01L 41/053 20060101 H01L041/053 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 23, 2015 |
JP |
2015-208802 |
Claims
1. An elastic wave device comprising: a supporting substrate; an
acoustic multilayer film on the supporting substrate; a
piezoelectric substrate on the acoustic multilayer film; and an IDT
electrode on the piezoelectric substrate; wherein an absolute value
of a thermal expansion coefficient of the piezoelectric substrate
is larger than an absolute value of a thermal expansion coefficient
of the supporting substrate; the acoustic multilayer film includes
at least four acoustic impedance layers; the at least four acoustic
impedance layers include at least one low acoustic impedance layer
and at least one high acoustic impedance layer having an acoustic
impedance higher than the low acoustic impedance layer; and a
bonding layer is provided at any position in a range of from inside
a first acoustic impedance layer of the at least four acoustic
impedance layers from the piezoelectric substrate side towards the
supporting substrate side, to an interface between a third acoustic
impedance layer and a fourth acoustic impedance layer of the at
least four acoustic impedance layers.
2. The elastic wave device according to claim 1, wherein the
bonding layer is provided inside one acoustic impedance layer
selected from the first acoustic impedance layer, the second
acoustic impedance layer, and the third acoustic impedance layer of
the at least four acoustic impedance layers from the piezoelectric
substrate side towards the supporting substrate side.
3. The elastic wave device according to claim 1, wherein the
bonding layer is provided at an interface between any two adjacent
acoustic impedance layers among the at least four acoustic
impedance layers from the piezoelectric substrate side toward the
supporting substrate side.
4. The elastic wave device according to claim 1, wherein a plate
wave of an S.sub.0 mode, an A.sub.0 mode, an A.sub.1 mode, an
SH.sub.0 mode, or an SH.sub.1 mode is used as a propagating elastic
wave.
5. The elastic wave device according to claim 1, wherein the
bonding layer has a thickness of about 5 nm or less.
6. The elastic wave device according to claim 1, wherein the
bonding layer also defines and functions as an insulating
layer.
7. The elastic wave device according to claim 1, wherein the
supporting substrate is made of glass or Si, and the piezoelectric
substrate is made of LiNbO.sub.3 or LiTaO.sub.3.
8. The elastic wave device according to claim 1, wherein the low
acoustic impedance layer is made of silicon oxide.
9. The elastic wave device according to claim 1, wherein the high
acoustic impedance layer is made of tungsten, platinum, tantalum,
silicon nitride, or aluminum nitride.
10. The elastic wave device according to claim 1, wherein a
thickness of each of the acoustic impedance layers is in a range of
about 1/4 of a thickness of the piezoelectric substrate to about 10
times the thickness of the piezoelectric substrate.
11. The elastic wave device according to claim 1, wherein the
bonding layer is made of a Ti oxide or a Ti nitride.
12. The elastic wave device according to claim 1, wherein the IDT
electrode is made of at least one of Al, Cu, Pt, Au, Ag, Ti, Ni,
Cr, Mo, or W, or an alloy primarily including any of Al, Cu, Pt,
Au, Ag, Ti, Ni, Cr, Mo, or W.
13. The elastic wave device according to claim 1, wherein the IDT
electrode is made of Al.
14. The elastic wave device according to claim 1, wherein the IDT
electrode is defined by a multilayer metal film including a Ti
layer and an AlCu that are stacked on one another.
15. The elastic wave device according to claim 1, further
comprising electrode lands provided on the piezoelectric
substrate.
16. The elastic wave device according to claim 15, wherein the
electrode layers are defined by a multilayer metal film including a
Ti layer and an Al layer that are stacked on one another.
17. The elastic wave device according to claim 1, wherein a
thickness of the IDT electrode is about 10 nm to about 2000 nm.
18. The elastic wave device according to claim 15, wherein a
thickness of each of the electrode lands is about 100 nm to about
2000 nm.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority to Japanese
Patent Application No. 2015-208802 filed on Oct. 23, 2015 and is a
Continuation Application of PCT Application No. PCT/JP2016/071725
filed on Jul. 25, 2016. The entire contents of each application are
hereby incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0002] The present invention relates to an elastic wave device for
use in a resonator, a bandpass filter, or other suitable
devices.
2. Description of the Related Art
[0003] Heretofore, elastic wave devices have been widely used in
resonators and bandpass filters. In such elastic wave devices,
various elastic waves, such as Rayleigh waves and shear horizontal
(SH) waves, are used.
[0004] In WO 2012/086441 A1, an elastic wave device that uses a
plate wave is disclosed. In the elastic wave device described in WO
2012/086441 A1, an acoustic reflector layer, a piezoelectric layer,
and an IDT electrode are stacked on a supporting substrate in that
order. In WO 2012/086441 A1, when producing the elastic wave
device, a piezoelectric is bonded to the supporting substrate with
the acoustic reflector layer stacked thereon.
[0005] In WO 2012/086639 A1, an elastic wave device in which a high
acoustic velocity film, a low acoustic velocity film, and a
piezoelectric film are stacked on a supporting substrate in that
order is disclosed. In WO 2012/086639 A1, when producing the
elastic wave device, the supporting substrate is bonded to a
multilayer body in which the piezoelectric film, the low acoustic
velocity film, and the high acoustic velocity film are stacked.
[0006] However, an elastic wave device obtained by a method
including bonding a piezoelectric to a supporting substrate on
which an acoustic reflector layer is stacked, as in WO 2012/086441
A1, may undergo deterioration of characteristics.
[0007] In contrast, when a supporting substrate and other elements
are bonded by the bonding method described in WO 2012/086639 A1,
many films must be formed on the piezoelectric film side. When many
films are formed on the piezoelectric film side, stress is
generated in the films, and the piezoelectric film may warp. Thus,
the characteristics of the elastic wave device described in WO
2012/086639 A1 also deteriorate in some cases.
SUMMARY OF THE INVENTION
[0008] Preferred embodiments of the present invention provide
elastic wave devices in each of which warpage of a piezoelectric
substrate and deterioration of the characteristics are unlikely to
occur.
[0009] An elastic wave device according to a preferred embodiment
of the present invention includes a supporting substrate, an
acoustic multilayer film on the supporting substrate, a
piezoelectric substrate on the acoustic multilayer film, and an IDT
electrode on the piezoelectric substrate. An absolute value of a
thermal expansion coefficient of the piezoelectric substrate is
larger than an absolute value of a thermal expansion coefficient of
the supporting substrate. The acoustic multilayer film includes at
least four acoustic impedance layers. The at least four acoustic
impedance layers include at least one low acoustic impedance layer
and at least one high acoustic impedance layer having an acoustic
impedance higher than the low acoustic impedance layer. The elastic
wave device further includes a bonding layer provided at any
position in a range of from inside the first acoustic impedance
layer from the piezoelectric substrate side towards the supporting
substrate side, to an interface between the third acoustic
impedance layer and the fourth acoustic impedance layer.
[0010] In an elastic wave device according to a preferred
embodiment of the present invention, the bonding layer is provided
inside one acoustic impedance layer selected from the first to
third acoustic impedance layers from the piezoelectric substrate
side towards the supporting substrate side.
[0011] In an elastic wave device according to a preferred
embodiment of the present invention, the bonding layer is provided
at an interface between any two adjacent acoustic impedance layers
among the first to fourth acoustic impedance layers from the
piezoelectric substrate side towards the supporting substrate
side.
[0012] In an elastic wave device according to a preferred
embodiment of the present invention, a plate wave of an S.sub.0
mode, an A.sub.0 mode, an A.sub.1 mode, an SH.sub.0 mode, or an
SH.sub.1 mode is used as a propagating elastic wave.
[0013] In an elastic wave device according to a preferred
embodiment of the present invention, the bonding layer has a
thickness of about 5 nm or less. In this case, deterioration of the
characteristics is further reduced or prevented.
[0014] In an elastic wave device according to a preferred
embodiment of the present invention, the bonding layer also defines
and functions as an insulating layer. In this case, deterioration
of the characteristics is further reduced or prevented.
[0015] In an elastic wave device according to a preferred
embodiment of the present invention, the supporting substrate is
made of glass or Si, and the piezoelectric substrate is made of
LiNbO.sub.3 or LiTaO.sub.3. In this case, warpage of the
piezoelectric substrate is further reduced or prevented.
[0016] In an elastic wave device according to a preferred
embodiment of the present invention, the low acoustic impedance
layer is made of silicon oxide. In this case, the plate wave is
able to be more efficiently confined.
[0017] In an elastic wave device according to a preferred
embodiment of the present invention, the high acoustic impedance
layer is made of tungsten, platinum, tantalum, silicon nitride, or
aluminum nitride. In this case, the plate wave is able to be more
efficiently confined.
[0018] According to preferred embodiments of the present invention,
elastic wave devices in each of which warpage of a piezoelectric
substrate and deterioration of the characteristics are unlikely to
occur are provided.
[0019] The above and other elements, features, steps,
characteristics and advantages of the present invention will become
more apparent from the following detailed description of the
preferred embodiments with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1A is a schematic elevational cross-sectional view of
an elastic wave device according to a first preferred embodiment of
the present invention, and FIG. 1B is a schematic plan view of an
electrode structure of the elastic wave device.
[0021] FIG. 2 is a partially cut-away schematic cross-sectional
view illustrating an enlarged relevant portion of the elastic wave
device of the first preferred embodiment of the present
invention.
[0022] FIG. 3 is a graph illustrating the relationship between the
thickness of the piezoelectric substrate and the impedance ratio
(Za/Zr) when the number of acoustic impedance layers that are
stacked to define the acoustic multilayer film is varied in
Experimental Example 1.
[0023] FIGS. 4A to 4D are each a schematic elevational
cross-sectional view illustrating a method for producing the
elastic wave device according to the first preferred embodiment of
the present invention.
[0024] FIG. 5 is a graph illustrating the relationship between the
number of acoustic impedance layers stacked and the amount of
warpage of the piezoelectric substrate when an X-cut-LiNbO.sub.3
substrate was used as the piezoelectric substrate in Experimental
Example 2.
[0025] FIG. 6 is a graph showing the relationship between the
bonding position of the bonding layer and the impedance ratio
(Za/Zr) in Experimental Example 3.
[0026] FIG. 7 is a graph illustrating resonance characteristics
when the bonding layer is provided on the first acoustic impedance
layer from the piezoelectric substrate side in experimental Example
3.
[0027] FIG. 8 is a partially cut-away schematic cross-sectional
view illustrating an enlarged relevant portion of an elastic wave
device of a second preferred embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] Preferred embodiments of the present invention will now be
described with reference to the drawings.
[0029] It should be noted that the preferred embodiments described
in this description are only exemplary, and partially substituting
or combining the features of different preferred embodiments is
possible.
First Preferred Embodiment
[0030] FIG. 1A is a schematic elevational cross-sectional view of
an elastic wave device according to a first preferred embodiment of
the present invention, and FIG. 1B is a schematic plan view of an
electrode structure thereof. FIG. 2 is a partially cut-away
schematic cross-sectional view illustrating an enlarged relevant
portion of the elastic wave device of the first preferred
embodiment of the present invention.
[0031] An elastic wave device 1 uses a plate wave of an S.sub.0
mode, an A.sub.0 mode, an A.sub.1 mode, an SH.sub.0 mode, an
SH.sub.1 mode, or other suitable mode. The elastic wave device 1
includes a supporting substrate 2. An acoustic multilayer film 3 is
stacked on the supporting substrate 2. A piezoelectric substrate 4
is stacked on the acoustic multilayer film 3. An IDT electrode 5
and electrode lands 6a and 6b are stacked on the piezoelectric
substrate 4. The electrode lands 6a and 6b are electrically coupled
to the IDT electrode 5.
[0032] The supporting substrate 2 is preferably made of Si, for
example. The material from which the supporting substrate 2 is made
is not particularly limited. Examples of the material that may be
used include piezoelectrics, such as sapphire, LiTaO.sub.3,
LiNbO.sub.3, and crystal; dielectrics, such as various ceramics and
glass, e.g., alumina, magnesia, silicon nitride, aluminum nitride,
silicon oxide, aluminum oxide, silicon carbide, zirconia,
cordierite, mullite, steatite, and forsterite; semiconductors, such
as silicon and gallium nitride; and resins.
[0033] The acoustic multilayer film 3 of the present preferred
embodiment includes low acoustic impedance layers 3a, 3c, 3e, and
3g, and high acoustic impedance layers 3b, 3d, and 3f. The acoustic
impedance of the high acoustic impedance layers 3b, 3d, and 3f is
higher than the acoustic impedance of the low acoustic impedance
layers 3a, 3c, 3e, and 3g. In the present preferred embodiment, the
low acoustic impedance layers 3a, 3c, 3e, and 3g and the high
acoustic impedance layers 3b, 3d, and 3f are alternately arranged
in the stacking direction. Thus, the plate wave propagating from
the piezoelectric substrate 4 is reflected at the interfaces
between the low acoustic impedance layers 3a, 3c, 3e, and 3g and
the high acoustic impedance layers 3b, 3d, 3f, the interfaces being
the upper surfaces of the low acoustic impedance layers 3a, 3c, 3e,
and 3g. In this manner, the energy of the plate wave is efficiently
confined.
[0034] In preferred embodiments of the present invention, the low
acoustic impedance layers 3a, 3c, 3e, and 3g and the high acoustic
impedance layers 3b, 3d, and 3f need not be arranged alternately in
the stacking direction. From the viewpoint of further improving the
plate wave confining efficiency, at least one of the low acoustic
impedance layers 3a, 3c, 3e, and 3g is preferably provided on the
piezoelectric substrate 4 side with respect to at least one of the
high acoustic impedance layers 3b, 3d, and 3f. More preferably, the
low acoustic impedance layers 3a, 3c, 3e, and 3g and the high
acoustic impedance layers 3b, 3d, and 3f are alternately arranged
in the stacking direction.
[0035] The low acoustic impedance layers 3a, 3c, 3e, and 3g and the
high acoustic impedance layers 3b, 3d, and 3f preferably have
acoustic impedance values that satisfy the relationship, high
acoustic impedance layer>low acoustic impedance layer, and the
materials therefor may be any as long as this relationship is
satisfied.
[0036] In the present preferred embodiment, the acoustic multilayer
film 3 preferably includes seven acoustic impedance layers, for
example. However, the number of acoustic impedance layers that are
stacked may be any as long as the number is 4 or more. The upper
limit of the number of the acoustic impedance layers stacked is not
particularly limited, but is preferably about 20, for example.
Since the elastic wave device includes an acoustic multilayer film
that includes at least four acoustic impedance layers, the plate
wave is able to be highly efficiently confined. This will now be
described in detail with reference to Experimental Example 1.
[0037] In Experimental Example 1, an elastic wave device 1, which
is a one-port-type elastic wave resonator, was prepared under the
following conditions, and, for example, a S.sub.0-mode plate wave
was excited.
[0038] Supporting substrate 2: Si Substrate
[0039] Acoustic multilayer film 3: number of layers stacked: 2, 4,
or 6, low acoustic impedance layer: SiO.sub.2, high acoustic
impedance layer: Pt, film thickness of each layer: SiO.sub.2: about
240 nm, Pt: about 150 nm
[0040] Piezoelectric substrate 4: X-cut --LiNbO.sub.3 {Euler angles
(90.degree., 90.degree., 40.degree.)}
[0041] IDT electrode 5: AlCu (Cu about 1%)/Ti, duty ratio: about
0.5, number of pairs of electrode fingers: 100, intersecting width:
about 25 .mu.m, wavelength (.lamda.) determined by electrode finger
pitch: about 1.7 .mu.m
[0042] FIG. 3 is a graph illustrating the relationship between the
thickness of the piezoelectric substrate 4 (LiNbO.sub.3) and the
impedance ratio (Za/Zr) when the number of acoustic impedance
layers that are stacked to define the acoustic multilayer film is
varied.
[0043] FIG. 3 demonstrates that good impedance characteristics are
obtained in the elastic wave devices 1 with four and six stacked
acoustic impedance layers as compared to the elastic wave device 1
with two stacked acoustic impedance layers. This shows that in
order to efficiently confine the plate wave and obtain good
impedance characteristics, at least four acoustic impedance layers
are preferable.
[0044] As described above, the acoustic multilayer film preferably
includes at least four acoustic impedance layers. In this manner,
the plate wave is able to be efficiently confined. The acoustic
multilayer film may further include an additional layer or layers,
such as a layer made of TiO.sub.2 or other suitable material, as
long as at least four acoustic impedance layers are included.
[0045] From the viewpoint of further efficiently confining the
energy of the plate wave, the thickness of each of the acoustic
impedance layers defining the acoustic multilayer film 3 is
preferably in the range of about 1/4 of the thickness of the
piezoelectric substrate 4 to about 10 times the thickness of the
piezoelectric substrate 4, for example. However, the thickness of
each of the acoustic impedance layers is not particularly
limited.
[0046] The low acoustic impedance layers 3a, 3c, 3e, and 3g are
preferably made of SiO.sub.2, for example. Alternatively, the low
acoustic impedance layers 3a, 3c, 3e, and 3g may be made of Al, Ti,
or other suitable material, for example.
[0047] The high acoustic impedance layers 3b, 3d, and 3f are
preferably made of Pt, for example. Alternatively, the high
acoustic impedance layers 3b, 3d, and 3f may be made of AlN, W,
LiTaO.sub.3, Al.sub.2O.sub.3, LiNbO.sub.3, Ta, SiN, ZnO, or other
suitable material, for example.
[0048] As illustrated in the enlarged view of FIG. 2, in the
present preferred embodiment, a bonding layer 9 is provided at the
interface between the low acoustic impedance layer 3e and the high
acoustic impedance layer 3d. In other words, the bonding layer 9 is
provided at the interface between the acoustic impedance layer 3e,
which is the third acoustic impedance layer from the piezoelectric
substrate 4 side towards the supporting substrate 2 side, and the
acoustic impedance layer 3d, which is the fourth acoustic impedance
viewed in the same manner. Thus, in the present preferred
embodiment, the bonding layer 9 is not provided directly below the
piezoelectric substrate 4.
[0049] The bonding layer 9 is preferably made of a Ti oxide, for
example. Thus, in the present preferred embodiment, the bonding
layer 9 is an insulating layer. Note that the material for the
bonding layer 9 is not limited to the Ti oxide, and may be an oxide
of other metals, such as Al, for example. Alternatively, instead of
a metal oxide, metal, such as Ti or Al, for example, may be used
for the bonding layer 9. In order to enable electrical insulation,
a metal oxide or a metal nitride is preferable. In particular, an
oxide or nitride of Ti is preferable for its high bonding
strength.
[0050] The thickness of the bonding layer 9 may be any thickness,
but is preferably about 5 nm or less, for example. When the bonding
layer 9 is an insulating layer and the thickness of the bonding
layer 9 is set within the above-described range as in the present
preferred embodiment, deterioration of the characteristics of the
elastic wave device 1 is further reduced or prevented.
[0051] The piezoelectric substrate 4 is preferably made of
LiNbO.sub.3, for example. Thus, the absolute value of the thermal
expansion coefficient of the piezoelectric substrate 4 is larger
than the absolute value of the thermal expansion coefficient of the
supporting substrate 2 made of Si. The piezoelectric substrate 4 is
not particularly limited as long as the absolute value of the
thermal expansion coefficient is larger than that of the supporting
substrate 2. A substrate made of a different piezoelectric single
crystal, such as LiTaO.sub.3, or a substrate made of a
piezoelectric ceramic, for example, may be used as the
piezoelectric substrate 4.
[0052] However, as in the present preferred embodiment, a substrate
made of glass or Si is preferably used as the supporting substrate
2, and a substrate made of LiNbO.sub.3 or LiTaO.sub.3 is preferably
used as the piezoelectric substrate 4. In this case, because the
difference in the absolute value of the thermal expansion
coefficient is decreased, warpage of the piezoelectric substrate 4
is further reduced or prevented.
[0053] Although FIG. 1A only schematically shows an electrode
structure, an electrode structure illustrated in FIG. 1B is
preferably provided on the piezoelectric substrate 4. In other
words, the IDT electrode 5 and reflectors 7 and 8, which are
located on both sides of the IDT electrode 5 in the elastic wave
propagation direction, are provided. As a result, a one-port-type
elastic wave resonator is provided. However, the reflectors 7 and 8
are optional.
[0054] As illustrated in FIG. 1B, the IDT electrode 5 includes a
first bus bar, a second bus bar, a plurality of first electrode
fingers, and a plurality of second electrode fingers. The first
electrode fingers and the second electrode fingers are
interdigitated. The first electrode fingers are connected to the
first bus bar, and the second electrode fingers are connected to
the second bus bar.
[0055] When an alternating voltage is applied to the IDT electrode
5, the portion of the piezoelectric substrate 4 in which the IDT
electrode 5 is provided is excited. The elastic wave device 1 uses
the plate wave as the elastic wave generated by excitation of the
IDT electrode 5 as described above.
[0056] Although omitted from the drawings of the present preferred
embodiment, a SiO.sub.2 film or a SiN film, which defines and
functions as a temperature adjusting film, may be provided to cover
the IDT electrode 5.
[0057] The IDT electrode 5 and the electrode lands 6a and 6b are
preferably made of Al in the present preferred embodiment. The IDT
electrode 5 and the electrode lands 6a and 6b may be made of any
appropriate metal, such as Al, Cu, Pt, Au, Ag, Ti, Ni, Cr, Mo, or
W, or an alloy primarily including any of these metals, for
example. The IDT electrode 5 and the electrode lands 6a and 6b may
be made of a multilayer metal film obtained by stacking a plurality
of metal films.
[0058] Since the number of acoustic impedance layers stacked in the
elastic wave device 1 of the present preferred embodiment is at
least 4, the plate wave is efficiently confined. Furthermore, in
the elastic wave device 1, the bonding layer 9 is provided at the
interface between the acoustic impedance layer 3e, which is the
third acoustic impedance layer from the piezoelectric substrate 4
side towards the supporting substrate 2 side, and the acoustic
impedance layer 3d, which is the fourth acoustic impedance layer
viewed in the same manner. Thus, when the piezoelectric substrate 4
is bonded to the supporting substrate 2 during production, warpage
of the piezoelectric substrate 4 is unlikely to occur. Moreover,
warpage of the piezoelectric substrate 4 in the elastic wave device
1 obtained as a final product is also unlikely to occur. Thus,
deterioration of the characteristics rarely occurs. This will now
be specifically described by describing a production method.
[0059] Although the method for producing the elastic wave device 1
is not particularly limited, one non-limiting example of a method
according to a preferred embodiment of the present invention is
described with reference to FIGS. 4A to 4D.
[0060] First, as illustrated in FIG. 4A, a piezoelectric substrate
4A and a supporting substrate 2 are prepared. A low acoustic
impedance layer 3g preferably formed of SiO.sub.2 is formed on one
main surface of the piezoelectric substrate 4A. Then, a high
acoustic impedance layer 3f preferably formed of SiN and a low
acoustic impedance layer 3e preferably formed of SiO.sub.2 are
sequentially stacked in that order on the low acoustic impedance
layer 3g. As a result, a multilayer film is formed on the
piezoelectric substrate 4A.
[0061] Two low acoustic impedance layers 3a and 3c formed of
SiO.sub.2 and two high acoustic impedance layers 3b and 3d formed
of SiN are alternately stacked on one main surface of the
supporting substrate 2 starting from the low acoustic impedance
layer 3a formed of SiO.sub.2. As a result, a multilayer film is
formed on the supporting substrate 2.
[0062] At least the high acoustic impedance layer 3d formed of SiN
is to be formed as the topmost layer of the multilayer film on the
supporting substrate 2. In this case, the elastic wave device 1
including four acoustic impedance layers is obtained when combined
with three acoustic impedance layers on the piezoelectric substrate
4A. Acoustic impedance layers may be formed between the supporting
substrate 2 and the high acoustic impedance layer 3d as in the
present preferred embodiment. Alternatively, an additional layer or
layers may be formed between the supporting substrate 2 and the
high acoustic impedance layer 3d. An example of the additional
layer may preferably be a layer formed of TiO.sub.2.
[0063] A plate made of LiNbO.sub.3, for example, is preferably used
as the piezoelectric substrate 4A. However, a substrate made of a
different piezoelectric single crystal, such as LiTaO.sub.3, or a
substrate composed of a piezoelectric ceramic, for example, may be
used as the piezoelectric substrate 4A.
[0064] Silicon (Si) is preferably used in the supporting substrate
2. However, piezoelectrics such as sapphire, LiTaO.sub.3,
LiNbO.sub.3, and crystal, dielectrics such as various ceramics and
glass, such as, alumina, magnesia, silicon nitride, aluminum
nitride, silicon oxide, aluminum oxide, silicon carbide, zirconia,
cordierite, mullite, steatite, and forsterite, and semiconductors
such as silicon and gallium nitride, resins, for example, may be
used in the supporting substrate 2.
[0065] The low acoustic impedance layers 3a, 3c, 3e, and 3g and
high acoustic impedance layers 3b, 3d, and 3f may be formed by a
sputtering method, a vapor deposition method, a CVD method, or
other suitable method, for example. The thickness of each of the
low acoustic impedance layers 3a, 3c, 3e, and 3g and the high
acoustic impedance layers 3b, 3d, and 3f is not particularly
limited, and may preferably be about 50 nm to about 2000 nm, for
example. The acoustic impedance layer may be subjected to
patterning as appropriate.
[0066] Next, the surface of the low acoustic impedance layer 3e,
which will form the bonding surface for the multilayer film stacked
on the piezoelectric substrate 4A, and the surface of the high
acoustic impedance layer 3d, which will form the bonding surface
for the multilayer film stacked on the supporting substrate 2, are
polished. After polishing, as illustrated in FIG. 4B, the
piezoelectric substrate 4A and the supporting substrate 2, on each
of which the multilayer film is formed, are bonded to each other.
For bonding the piezoelectric substrate 4A and the supporting
substrate 2 to each other, a bonding film, not illustrated, in the
drawing and preferably made of Ti for forming the bonding layer 9
is interposed between the low acoustic impedance layer 3e, which
forms the topmost surface of the multilayer film on the
piezoelectric substrate 4A, and the high acoustic impedance layer
3d, which forms the topmost surface of the multilayer film on the
supporting substrate 2, and diffusion bonding is preferably
performed to achieve bonding. The bonding method may be hydrophilic
bonding or activated bonding.
[0067] Next, as illustrated in FIG. 4C, the thickness of the
piezoelectric substrate 4A is reduced so that a plate wave is
excitable so as to obtain the piezoelectric substrate 4. From the
viewpoint of the plate wave excitation efficiency, the thickness of
the piezoelectric substrate 4 is preferably about 1 .mu.m or less,
for example.
[0068] After the thickness of the piezoelectric substrate 4A is
reduced, preferably, a heat treatment is performed at a temperature
of about 300.degree. C., for example, so that the bonding film made
of Ti described above is oxidized so that an insulating property is
provided.
[0069] Lastly, as illustrated in FIG. 4D, an IDT electrode 5 and
electrode lands 6a and 6b are formed on a main surface of the
piezoelectric substrate 4, the main surface being on the opposite
side from the acoustic multilayer film 3. As a result, the elastic
wave device 1 is obtained.
[0070] The IDT electrode 5 and the electrode lands 6a and 6b may
preferably be formed by a vapor-deposition lift-off method, for
example. The thickness of the IDT electrode 5 is not particularly
limited but may preferably be about 10 nm to about 2000 nm, for
example. The thickness of the electrode lands 6a and 6b is not
particularly limited but may preferably be about 100 nm to about
2000 nm, for example.
[0071] In the present preferred embodiment, the IDT electrode 5 is
preferably formed of a multilayer metal film prepared by stacking
Ti and AlCu (Cu 1%) in this order. The electrode lands 6a and 6b
are each preferably formed of a multilayer metal film prepared by
stacking Ti and Al in this order.
[0072] According to the production method of the present preferred
embodiment, only three acoustic impedance layers are stacked on the
piezoelectric substrate 4A, and, thus, large film stress does not
act on the piezoelectric substrate 4A. Thus, when bonded to the
supporting substrate 2, the piezoelectric substrate 4A rarely
undergo warpage. This will be described through concrete
Experimental Example 2.
[0073] In Experimental Example 2, in preparing the elastic wave
device 1 by the production method described above, the number of
acoustic impedance layers stacked on the piezoelectric substrate 4A
is changed from 1 to 6.
[0074] FIG. 5 is a graph illustrating the relationship between the
number of stacked acoustic impedance layers and the amount of
warpage of the piezoelectric substrate when an X-cut-LiNbO.sub.3
substrate was used as the piezoelectric substrate. In FIG. 5, the
number of stacked acoustic impedance layers is the number of
acoustic impedance layers that are stacked on the piezoelectric
substrate 4A before being bonded to the supporting substrate 2. The
amount of warpage is the amount of warpage of the piezoelectric
substrate 4A having a diameter of about 4 inches when the
piezoelectric substrate 4A is bonded to the supporting substrate
2.
[0075] As illustrated in FIG. 5, the amount of warpage decreases as
the number of acoustic impedance layers that are stacked on the
piezoelectric substrate 4A decreases. In particular, when the
number of stacked acoustic impedance layers is 3 or less, the
amount of warpage of the piezoelectric substrate 4A remains within
the range of about 150 .mu.m or less, which is the range in which
issues rarely arise in the bonding of the substrates, in the Y axis
direction of the piezoelectric substrate 4A (LiNbO.sub.3 substrate)
as well as in the Z axis direction of the piezoelectric substrate
4A (LiNbO.sub.3 substrate). In contrast, when the number of layers
stacked is 4 or more, the amount of warpage in the Y axis direction
is greater than about 150 .mu.m. This shows that when four or more
acoustic impedance layers are to be stacked between the
piezoelectric substrate 4A and the supporting substrate 2, layers
up to and including the third layer are to be stacked on the
piezoelectric substrate 4A side, and other layers are to be stacked
on the supporting substrate 2 side.
[0076] As such, when bonding the piezoelectric substrate 4A and the
supporting substrate 2 to each other, the number of acoustic
impedance layers that are stacked on the piezoelectric substrate 4A
side is preferably 3 or less. In other words, in the elastic wave
device 1 obtained as a product, the bonding layer 9 is preferably
provided at any position in a range up to and including the
interface between the third acoustic impedance layer 3e and the
fourth acoustic impedance layer 3d from the piezoelectric substrate
4 side towards the supporting substrate 2 side. In such a case,
since a large film stress does not act on the piezoelectric
substrate 4A, warpage of the piezoelectric substrate 4A is unlikely
to occur when the piezoelectric substrate 4A and the supporting
substrate 2 are being bonded to each other. As a result, the
piezoelectric substrate 4A and the supporting substrate 2 are able
to be easily bonded to each other. Moreover, since large film
stress does not act on the piezoelectric substrate 4A, stress
acting on the piezoelectric substrate 4A after thickness reduction
also decreases, and warpage of the piezoelectric substrate 4 after
thickness reduction is also able to be reduced or prevented. Thus,
in a preferred embodiment of the present invention, a piezoelectric
substrate 4 warpage canceling step is not necessary. From the
viewpoint of further reducing or preventing warpage of the
piezoelectric substrate 4, the position of the bonding layer 9 is
preferably as close to the piezoelectric substrate 4 as
possible.
[0077] Note that, in the elastic wave device 1, the bonding layer 9
is not provided directly below the piezoelectric substrate 4. In
other words, in the elastic wave device 1, the bonding layer 9 is
absent at the interface between the piezoelectric substrate 4 and
the low acoustic impedance layer 3g. Thus, the elastic wave device
1 is unlikely to undergo deterioration of the characteristics. This
will now be described through Experimental Example 3.
[0078] In Experimental Example 3, an elastic wave device 1, which
is a one-port-type elastic wave resonator, was prepared under the
following conditions, and a S.sub.0-mode plate wave was
excited.
[0079] Supporting substrate 2: Si substrate
[0080] Low acoustic impedance layer 3a: SiO.sub.2, film thickness:
0.4.lamda.
[0081] High acoustic impedance layer 3b: SiN, film thickness:
0.11.lamda.
[0082] Low acoustic impedance layer 3c: SiO.sub.2, film thickness:
0.1.lamda.
[0083] High acoustic impedance layer 3d: SiN, film thickness:
0.11.lamda.
[0084] Low acoustic impedance layer 3e: SiO.sub.2, film thickness:
0.1.lamda.
[0085] High acoustic impedance layer 3f: SiN, film thickness:
0.11.lamda.
[0086] Low acoustic impedance layer 3g: SiO.sub.2, film thickness:
0.1.lamda.
[0087] Bonding layer 9: epoxy resin, film thickness: about
0.05.lamda.
[0088] Piezoelectric substrate 4: LiNbO.sub.3 {Euler angles
(90.degree., 90.degree., 40.degree.)}, film thickness: about
0.2.lamda.
[0089] IDT electrode 5: Al, film thickness: about 0.07.lamda., duty
ratio: about 0.5, wavelength (.lamda.) determined by electrode
finger pitch: about 1.0 .mu.m
[0090] FIG. 6 is a graph showing the relationship between the
bonding position of the bonding layer and the impedance ratio
(Za/Zr). In the graph, the bonding position of the bonding layer 9
indicates the ordinal number of the acoustic impedance layer on
which the bonding layer 9 is provided as viewed from the
piezoelectric substrate 4 towards the supporting substrate 2. For
example, if the layer is the 0-th layer, the bonding layer 9 is
provided at the interface between the piezoelectric substrate 4 and
the low acoustic impedance layer 3g.
[0091] As illustrated in FIG. 6, as compared to when the bonding
position is at the 0-th layer, the characteristics are improved
when the bonding position is on the supporting substrate 2 side of
the first layer from the piezoelectric substrate 4 side (that is,
the bonding position is at the second or onward layer from the
piezoelectric substrate 4).
[0092] FIG. 7 is a graph illustrating resonance characteristics
when the bonding layer is provided on the first acoustic impedance
layer from the piezoelectric substrate side. In the graph, the
solid line indicates the results of providing the bonding layer 9
on the acoustic impedance layer 3g, which is the first layer from
the piezoelectric substrate 4 side. The broken line indicates the
results of a comparative example in which the bonding layer 9 is
provided on the 0-th acoustic impedance layer. In other words, the
results obtained when the bonding layer 9 is provided at the
interface between the piezoelectric substrate 4 and the first
acoustic impedance layer 3g are shown.
[0093] As illustrated in FIG. 7, providing the bonding layer 9 on
the acoustic impedance layer 3g, which is the first layer from the
piezoelectric substrate 4 side, reduces or prevents the response of
a spurious wave, which is indicated by arrow B1 and observed in the
comparative example, near the principal wave. The graph also
illustrates that the response of the spurious wave indicated by
arrow A, the response corresponding to the response of the spurious
wave of the comparative example indicated by arrow B2, is remote
from the principal wave. This shows that when the bonding layer 9
is provided on the supporting substrate 2 side with respect to the
interface between the piezoelectric substrate 4 and the first
acoustic impedance layer 3g, good characteristics are obtained.
[0094] Thus, it has been discovered that as compared to when the
bonding position is at the 0-th layer from the piezoelectric
substrate 4 side, the characteristics are improved when the bonding
position is in the first layer from the piezoelectric substrate 4
side, at the interface between the first and second layers, or on
the supporting substrate 2 side with respect to the first
layer.
[0095] Furthermore, as described above, in a preferred embodiment
of the present invention, when bonding the piezoelectric substrate
4A and the supporting substrate 2 to each other, the amount of
warpage of the piezoelectric substrate 4A is decreased because the
number of acoustic impedance layers stacked on the piezoelectric
substrate 4A side is 3 or less.
[0096] In view of the above, in the present preferred embodiment,
the characteristics and the amount of warpage of the piezoelectric
substrate are both improved by setting the bonding position to be
in the range of from the inside of the first acoustic impedance
layer from the piezoelectric substrate to the interface between the
third acoustic impedance layer and the fourth acoustic impedance
layer.
Second Preferred Embodiment
[0097] FIG. 8 is a partially cut-away schematic cross-sectional
view illustrating an enlarged relevant portion of the elastic wave
device of a second preferred embodiment of the present invention.
As illustrated in FIG. 8, in the second preferred embodiment, the
low acoustic impedance layer 3g has a structure obtained by bonding
a low acoustic impedance layer segment 3g1 and a low acoustic
impedance layer segment 3g2 to each other with the bonding layer 9.
Thus, the bonding layer 9 is provided inside the low acoustic
impedance layer 3g. The low acoustic impedance layer segment 3g1
and the low acoustic impedance layer segment 3g2 may preferably be
made of the same material as that for the low acoustic impedance
layers 3a, 3c, and 3e. Other features are the same or substantially
the same as those of the first preferred embodiment.
[0098] The elastic wave device according to the second preferred
embodiment may be produced by the same production method as that in
the first preferred embodiment. Specifically, the low acoustic
impedance layer segment 3g2 is stacked on the piezoelectric
substrate 4A, and other portions are stacked on the supporting
substrate 2. Then, the low acoustic impedance layer segment 3g2 on
the piezoelectric substrate 4A and the low acoustic impedance layer
segment 3g1, which is the topmost layer of the multilayer film on
the supporting substrate 2, are bonded together by the same or
substantially the same method as in the first preferred embodiment
to produce the elastic wave device.
[0099] In the second preferred embodiment, since the bonding layer
is provided inside the first acoustic impedance layer from the
piezoelectric substrate side towards the supporting substrate side,
warpage of the piezoelectric substrate and deterioration of the
characteristics are unlikely to occur.
[0100] As in the elastic wave device of the second preferred
embodiment, the bonding layer 9 may be provided inside any of the
first to third acoustic impedance layers from the piezoelectric
substrate 4 side towards the supporting substrate 2 side.
Furthermore, as in the elastic wave device of the first preferred
embodiment, the bonding layer 9 may be provided at the interface
between any two adjacent acoustic impedance layers among the first
to fourth acoustic impedance layers from the piezoelectric
substrate 4 side towards the supporting substrate 2 side. As
described above, in preferred embodiments of the present invention,
since the bonding layer is provided at any position in the range of
from inside the first acoustic impedance layer, which is the first
layer from the piezoelectric substrate side towards the supporting
substrate side, to the interface between the third and fourth
acoustic impedance layers, warpage of the piezoelectric substrate
and deterioration of the characteristics is unlikely to occur.
[0101] Elastic wave devices of preferred embodiments of the present
invention are widely used in various electronic appliances and
communication appliances. An example of the electronic appliances
is a sensor. Examples of the communication appliances include a
duplexer that includes an elastic wave device according to a
preferred embodiment of the present invention, a communication
module appliance that includes an elastic wave device according to
a preferred embodiment of the present invention, and a power
amplifier (PA) and/or a low noise amplifier (LNA) and/or a switch
(SW), and a mobile communication appliance and a healthcare
communication appliance that include the communication module
appliance. Examples of the mobile communication appliance include
cellular phones, smart phones, and car navigation systems. Examples
of the healthcare communication appliance include a body weight
meter and a body fat meter. The healthcare communication appliance
and the mobile communication appliance each include an antenna, an
RF module, an LSI, a display, an input unit, a power supply, and
other components.
[0102] While preferred embodiments of the present invention have
been described above, it is to be understood that variations and
modifications will be apparent to those skilled in the art without
departing from the scope and spirit of the present invention. The
scope of the present invention, therefore, is to be determined
solely by the following claims.
* * * * *